This invention relates to a process and a device for a spray treatment of polymer dispersions to reduce the content of monomers thereof.
The monomer content of polymer dispersions should be as low as possible for various reasons. On the one hand, during storage of the dispersions the monomers still present can react in uncontrolled manner and thus detrimentally affect the properties of the dispersions and the coatings made therefrom. On the other hand, monomers evaporating with the water when the dispersions are dried may be injurious to health. Especially in the processing of dispersions to obtain plastics powders, for example in the industrial production of polyvinyl chloride, considerable monomer amounts escape with the drying air if the dispersions have a high monomer content, for example above 0.1% by weight, which must be reduced because of environmental protection. Moreover, packing material for foodstuff made from dispersions having a high monomer content may alter the odor and taste of the packed goods so that they may become unpalatable.
Various processes have been proposed to reduce the monomer content of aqueous polymer dispersions.
In general, the monomers are removed by pressure release of the polymerization mixture.
It is also known to free polymer dispersions of monomers by passing through steam or inert gases. To this effect considerable amounts of steam or gas are required which cause difficulties, above all in the case of dispersions tending to foam. The addition of defoamers increases the expenses, is not always effective and may detrimentally affect the properties of the dispersions. To avoid the aforesaid difficulties it has also been proposed to destroy, by a rapid pressure reduction, the foam formed during the passage of steam at a speed of more than 100 m/sec, to separate the steam from the broken dispersion foam and to add the foam to the boiling dispersion. This process is carried out in continuous manner, the steam consumption is high and the expenditure pertaining to apparatus is considerable.
According to another known process the dispersion is mixed with steam or hot inert gases in a tube and after a short period of time the three phase mixture is divided again into dispersion and gaseous phase. It has also been proposed to atomize polymer dispersions in steam flowing at a high speed, to pass the mixture between heated plates to a release zone where the dispersion is separated from the gaseous phase is a cyclone. The two latter processes are carried out continuously but also require high amounts of steam and considerable expenditures pertaining to apparatus. Inspite of the high velocity of flow, the dispersion is easily overheated, and coagulations and backing of the polymers on the walls of the apparatus may occur. Moreover, monomers included in the polymer core, which require a certain diffusion time, are removed to an insufficient extent only. In most cases a prior degassing is necessary.
In another known process the dispersion is conducted in a tube in counter current flow with steam. In this case, too, the expenditure pertaining to apparatus is relatively high and with high passage rates dispersions with low surface tension start to foam. Moreover, deposits of solids may readily cause clogging.
Finally, it has been proposed to spray the dispersion into a steam atmosphere in such a manner that the sprayed particles fall through the steam owing to gravity. For this process a large and relatively expensive vessel is required.